AIMS
1. To determine the drying rate for different materials and the
moisture content and its relationship.2. To compare tray drier
process with other type of dying equipment.
THEORY
Drying is the mass transfer process involving the removal of
water or another solvent by evaporation from a solid, semi-solid or
liquid. This process is often used as a final production step
before selling or packaging product. A source of heat and an agent
to remove the vapor produced by the process are often involved.
There are two options of drying using the mechanism of heat
transfer that is through convection by direct driers or conduction
by contact or indirect driers. Depending on whether the drying
temperature is higher or lower than the boiling point of the liquid
to be separated, an additional distinction is made which are the
evaporating drying and vaporization drying. Drying can also be
classified either as a batch process as the materials is inserted
into the drying equipment and the process proceed for a given
period of time or as a continuous process, where the material is
continuously added to the dryer and dried material is continuously
removed.
In most cases, a gas stream for example air, applies the heat by
convection and carries away the vapor as humidity. Other cases are
vacuum drying where heat is supplied by conduction or radiation
while the vapor thus produced is removed by the vacuum system.
Another indirect technique is drum drying where a heated surface is
used to provide the energy and aspirators draw the vapor outside
the room.
For material that is difficult to dry, the heat transfer is
achieved by flowing the gas through a bulk of material to be dried
or a fluid bed. The air is not only used to feed in the heat but
also to absorb the vapor arising during the drying process. When it
flows into the drier, the air must therefore be hot as possible so
that it can absorb as much moisture as it can.
The most important key figure in drying is the moisture content
X of the material to be dried which can be defined as :X =
The change in the moisture content during the drying process can
be used to derive the drying rate V.V =
